The transformation-related protein 53 (TP53) gene encodes a 53 KDa protein—p53. The tumour suppressor protein p53 reacts to cellular stresses, such as hypoxia, DNA damage and oncogenic activation, via a number of posttranslational modifications including phosphorylation, acetylation and methylation, and acts as a signalling node in the diverse pathways that become activated. p53 has additional roles in other physiological processes, including autophagy, cell adhesion, cell metabolism, fertility, and stem cell aging and development. Phosphorylation of p53, resulting from activation of kinases including ATM, CHK1 and 2, and DNA-PK, results in a stabilised and transcriptionally active form of the protein, thus producing a range of gene products. The responses to p53 activation include apoptosis, survival, cell-cycle arrest, DNA-repair, angiogenesis, invasion and autoregulation. The specific combination of which, in concert with the cell's genetic background, gives rise to the observed cellular effect i.e. apoptosis, cell-cycle arrest or senescence. For tumour cells, the apoptotic pathway may be favoured due to the loss of tumour suppressor proteins and associated cell cycle checkpoint controls, coupled with oncogenic stress.
Under conditions of stress such as hypoxia and DNA damage it is known that the cellular level of the protein p53 increases. p53 is known to initiate transcription of a number of genes which govern progression through the cell cycle, the initiation of DNA repair and programmed cell death. This provides a mechanism for the tumour suppressor role of p53 evidenced through genetic studies.
The activity of p53 is negatively and tightly regulated by a binding interaction with the MDM2 protein, the transcription of which is itself directly regulated by p53. p53 is inactivated when its transactivation domain is bound by the MDM2 protein. Once inactivated the functions of p53 are repressed and the p53-MDM2 complex becomes a target for ubiquitinylation.
In normal cells the balance between active p53 and inactive MDM2-bound p53 is maintained in an autoregulatory negative feedback loop. That is to say that p53 can activate MDM2 expression, which in turn leads to the repression of p53.
It has been found that inactivation of p53 by mutation is common in around half of all common adult sporadic cancers. Furthermore, in around 10% of tumours, gene amplification and over-expression of MDM2 results in the loss of functional p53, thereby allowing malignant transformation and uncontrolled tumour growth.
Inactivation of p53 by a range of mechanisms is a frequent causal event in the development and progression of cancer. These include inactivation by mutation, targeting by oncogenic viruses and, in a significant proportion of cases, amplification and/or an elevated rate of transcription of the MDM2 gene resulting in overexpression or increased activation of the MDM2 protein. Gene amplification of MDM2 giving rise to overexpression of MDM2 protein has been observed in tumour samples taken from common sporadic cancers. Overall, around 10% of tumours had MDM2 amplification, with the highest incidence found in hepatocellular carcinoma (44%), lung (15%), sarcomas and osteosarcomas (28%), and Hodgkin disease (67%) (Danovi et al., Mol. Cell. Biol. 2004, 24, 5835-5843, Toledo et al., Nat Rev Cancer 2006, 6, 909-923, Gembarska et al., Nat Med 2012, 18, 1239-1247). Normally, transcriptional activation of MDM2 by activated p53 results in increased MDM2 protein levels, forming a negative feedback loop. The essential nature of p53 regulation by MDM2 and MDMX is demonstrated by gene knockout mouse models. MDM2−/− knockout mice are embryonically lethal around the time of implantation. Lethality is rescued in the double knockout for Mdm2 and Trp53. MDM2 inhibits the activity of p53 directly, by binding to and occluding the p53 transactivation domain, and by promoting the proteosomal destruction of the complex, through its E3-ubiquitin ligase activity. In addition, MDM2 is a transcriptional target of p53, and so the two proteins are linked in an autoregulatory feedback loop, ensuring that p53 activation is transient.
The induction of the p14ARF protein, the alternate reading frame (ARF) product of the p16INK4a locus, is also a mechanism of negatively regulating the p53-MDM2 interaction. p14ARF directly interacts with MDM2 and leads to up-regulation of p53 transcriptional response. Loss of p14ARF by a homozygous mutation in the CDKN2A (INK4A) gene will lead to elevated levels in MDM2 and, therefore, loss of p53 function and cell cycle control.
Although MDMX shows strong amino acid sequence and structural homology to MDM2, neither protein can substitute for loss of the other; MDMX null mice die in utero, whereas MDM2 knockout is lethal during early embryogenesis, however both can be rescued by p53 knockout, demonstrating p53-dependence of the lethality. MDMX also binds p53 and inhibits p53-dependent transcription, but unlike MDM2 it is not transcriptionally activated by p53 and so does not form the same autoregulatory loop. Furthermore, MDMX has neither E3 ubiquitin ligase activity nor a nuclear localisation signal, however it is believed to contribute to p53 degradation by forming heterodimers with MDM2 and contributing to MDM2 stabilisation.
The therapeutic rationale for MDM2-p53 inhibition is that a potent inhibitor of the protein-protein interaction will liberate p53 from the repressive control of MDM2, and activate p53 mediated cell death in the tumour. In tumours, selectivity is envisioned to result from p53 sensing preexisting DNA-damage or oncogenic activation signals that had previously been blocked by the action of MDM2 at normal or overexpressed levels. In normal cells, p53 activation is anticipated to result in activation of non-apoptotic pathways and if anything a protective growth inhibition response. In addition due to the non-genotoxic mechanism of action for MDM2-p53 inhibitors they are suitable for the treatment of cancer in particular in the pediatric population.
About 50% of cancers harbour cells in which TP53, the gene that encodes for p53, is mutated resulting in a loss of the protein's tumour suppressor function and sometimes even in p53 protein versions that gain novel oncogenic functions.
Cancers where there is a high level of MDM2 amplification include liposarcoma (88%), soft tissue sarcoma (20%), osteosarcoma (16%) oesophageal cancer (13%), and certain paediatric malignancies including B-cell malignancies.
The present invention describes a novel series of compounds which selectively inhibit the MDM2-p53 interaction and which have anticancer activity.